1. Field of the Invention
The present invention relates to an apparatus for applying cleaning process to various substrates such as a semiconductor wafer, a glass substrate for a liquid crystal display, a PDP (Plasma Display Panel) substrate or a glass substrate and a ceramic substrate for magnetic discs.
2. Description of the Background Art
A process of manufacturing semiconductor devices includes a step of forming a thin film on a semiconductor wafer (hereinafter referred to as “substrate”) and then selectively removing the thin film through etching process, which is repeated to obtain fine patterns on the substrate. Respective surfaces of the substrate, in particular, the major surface of the substrate on which thin films are to be formed, should be clean to precisely obtain the fine patterns on the substrate. Accordingly, a step of cleaning the substrate is conducted as required.
In a conventional substrate cleaning apparatus, a droplet-injection cleaning system, which uses a cleaning double fluids nozzle for strongly removing contaminants adhering the surface of the substrate, has been proposed.
FIG. 6 is a schematic drawing of a cleaning apparatus in which the conventional cleaning double fluids nozzle is used. This cleaning apparatus is provided with a cleaning cup 51, a spin chuck 52 for holding a substrate W inside the cleaning cup 51, an electric motor 53 for rotating this spin chuck 52, a gas supplying means 55 for supplying pressurized gas to the cleaning double fluids nozzle 60 for discharging droplets onto the surface of the substrate W, and a liquid supplying means 56 for supplying a pressurized liquid to the cleaning double fluids nozzle 60. Moreover, a robot arm 57 for holding and shifting the cleaning double fluids nozzle 60 is installed.
FIG. 7 shows a cross-sectional view of the conventional cleaning double fluids nozzle 60. The cleaning double fluids nozzle is provided with a first tube path 61 through which gas is transmitted, and a second tube path 62 the tip of which is allowed to penetrate the side wall of the first tube path 61 from the outside of the first tube path 61, and extended to the inside of the first tube path 61 and through which a fluid is transmitted. The tip of the second tube path 62 is extended in the same direction as the direction in which the first tube path 61 is extended.
A substrate W is fixed to the spin chuck 52, and rotated at a predetermined number of revolutions. Pressurized gas is supplied from a gas supplying means 55 to the cleaning double fluids nozzle 60, and a pressurized liquid is supplied from a liquid supplying means 56 thereto, respectively. In the cleaning double fluids nozzle 60, the gas and the liquid are mixed with each other so that the liquid is changed to droplets in the form of mist. These droplets are accelerated by the gas flow inside the first tube path 61, and discharged from the tip of the first tube path 61. The atomized droplets thus discharged are made to collide with the surface of the substrate W, thereby removing the contaminants adhering to the surface of the substrate W.
However, in the above-mentioned cleaning apparatus, the gas and the liquid are mixed inside the cleaning double fluids nozzle 60. Therefore, when the flow rate of one fluid is attempted to be changed independently of the flow rate of the other fluid, the latter is also changed since the respective pressures interfere with each other inside the first tube path 61.
In other words, in the case when the gas flow rate is increased so as to increase the cleaning strength, since the pressure of the gas inside the first tube path 61 increases so that the flow rate of the liquid supplied from the second tube path 62 is suppressed. Consequently, the droplets to be discharged from the tip opening of the nozzle of the double fluids nozzle 60 tend to have cleaning strength different from the initial cleaning strength due to the suppressed liquid flow rate.
Consequently, fine particles such as dusts and slurries tend to remain on the surface of the substrate W, resulting in a serious problem of a reduced yield in the manufacturing process of the semiconductor device.
Moreover, in the above-mentioned cleaning apparatus, since the gas and the liquid are mixed inside the cleaning double fluids nozzle 60, dusts are generated because irregularities on the inner wall of the nozzle 60 are cut. The dusts may be also generated during the mixing operation. The substances resulting from the dried liquid and adhering to the inside of the nozzle 60 are taken off by the flow in the nozzle.